The present invention relates to the method for enhancing the recovery of petroleum from an oil bearing formation.
In the recovery of oil from reservoirs, the use of primary production techniques (i.e., the use of only the initial formation energy to recover the crude oil) followed by the secondary technique of water flooding, recovers only a portion of the original oil present in the formation.
Moreover, the use of certain enhanced oil recovery (EOR) techniques are also known in the art. These techniques can generally be classified as either a thermally based recovery technique, e.g., utilizing steam, or a gas drive method that can be operated in a miscible or non-miscible manner.
Methods which employ steam are effective in the enhanced recovery of oil because the steam heats the formation, lowers the viscosity of the oil and thus, enhances the flow of the oil towards a production well. Moreover, these methods have become preferred methods for enhanced oil recovery of low gravity, high viscosity oils, because steam can cost effectively provide heat to such oils.
However, in these steam based techniques, it is common that the steam will find short-cut pathways from the injection well to some of the producing wells, thus bypassing oil which is present in the zone between the injection well and the production well. Also, after the initial steam injection breakthrough at the production well, the steam injection preferentially follows the path of the breakthrough. These pathways can take the form of channels in the formation or of gravity override in the upper portion of the oil bearing stratum. Gravity override results from the lower density and viscosity of the steam vapor compared to the liquid oil and water. Thus, the total amount of the formation that is swept by the steam injection is limited.
Various methods have been proposed to mitigate the loss of steam flow and heating value in the formation. For example, a number of commercial surfactants have been injected along with the steam to create a steam-foam flood. The foam serves to physically block the volumes through which the steam is shortcutting and divert the flow of the steam into the unswept portion of the formation.
In addition, various inert and non-condensable gases have been added to the steam, both in the presence and absence of foaming surfactants in order to enhance and maintain the oil-driving force with the formulation.
Examples of the steam-foam processes can be found in U.S. Pat. Nos. 4,086,964; 4,445,573; 4,393,937; 4,161,217; and 4,085,800.
In particular, certain alkyl aromatic sulfonates have been employed as a surfactant with the steam-foam drive systems. See, for example, Canadian Patent 1,247,850.
In such recovery operations maintaining the surfactant in a state suitable for foam generation is important. In particular, precipitation, phase separation and the like are detrimental to producing a useable foam. For example, although many aqueous surfactant systems, such as those employing alkyl aromatic sulfonates, are generally considered adequate foamers, they also have a relatively low tolerance to multivalent cations. Such cations, and in particular divalent cations, e.g., calcium or magnesium salts, or the like, are commonly encountered in waters or brines in subterranean reservoirs. The multivalent cations tend to react with anionic surfactants or the components of anionic surfactant systems to form precipitates, to cause phase separations, or the like and severely inhibit foam formation.
A variety of solutions have been offered in an attempt to solve the multivalent cation intolerance problems associated with such surfactant systems. See for example, U.S. Pat. No. 4,820,429 which employs a surfactant such as alkylated diphenyl oxide surfactants; as well as U.S. Pat. No. 4,643,256 which involved the use of an alkylpolyalkoxyalkylene or alkylarylpolyalkoxyalkylene sulfonate surfactant.
However, the need still exists for a composition which is capable of preventing the undesirable precipitation of alkyl aromatic sulfonates, in particular C.sub.20 -C.sub.24 linear alkyl aromatic sulfonates and which has adequate thermal stability so as to be employed at the relatively high temperatures, e.g., 400.degree.-600.degree. C. which can be associated with enhanced recovery operations.